This invention relates to a noise reduction circuit which is incorporated in, for example, a television receiver and in which delay elements are used so as to reduce white noise included in a composite picture signal or in a luminance signal and a chromaticity signal obtained as a result of YC separation of a composite picture signal.
In IDTV (Improved Television) in which television pictures of a high quality are demanded, a noise reduction circuit has been put into practical use so as to achieve one of indispensable functions of the IDTV. In the IDTV, a composite picture signal is not directly processed but is separated into a luminance signal and a chromaticity signal (referred to hereinafter as YC separation), and these signals are separately processed so as to reproduce a picture of a high quality. In the noise reduction circuit too, the luminance signal and the chromaticity signal obtained as a result of the YC separation are separately processed for the purpose of noise reduction. In the case of the IDTV, it is customary to employ a noise reduction circuit of recursive type utilizing delay elements provided in an IDTV receiver. A prior art noise reduction circuit of recursive type utilizing such delay elements is disclosed in, for example, "Noise Reducer, Takahashi, The Journal of The Institute of Television Engineers of Japan, Vol. 33, No. 4, (1979), pp. 296-300.
Examples of a prior art YC separation circuit and prior art noise reduction circuits will now be described by reference to FIGS. 1, 2 and 3.
FIG. 1 is a block diagram of the prior art YC separation circuit. Referring to FIG. 1, the prior art YC separation circuit includes an input terminal 1 for a composite picture signal, delay elements 4 and 5 for delaying the input composite picture signal by a predetermined period of time respectively, an adder 6 adding the output signal of the delay element 5 to the composite picture signal applied to the input terminal 1, a factor-of-1/2 multiplier circuit 7 multiplying the output signal of the adder 6 by the factor of 1/2, a subtracter 8 subtracting the output signal of the factor-of-1/2 multiplier circuit 7 from the output signal of the delay element 4, a factor-of-1/2 multiplier circuit 9 multiplying the output signal of the subtracter 8 by the factor of 1/2, a band-pass filter 10 extracting a chromaticity signal component from the composite picture signal, a subtracter 12 subtracting the output signal of the band-pass filter 10 from the output signal of the delay element 4, a chromaticity signal output terminal 31 connected to the output terminal of the band-pass filter 10, and a luminance signal output terminal 32 connected to the output terminal of the subtracter 12.
FIGS. 2 and 3 show the examples of the prior art noise reduction circuits. That is, FIG. 2 is a block diagram of the prior art noise reduction circuit for the luminance signal, and FIG. 3 is a block diagram of the prior art noise reduction circuit for the chromaticity signal.
Referring to FIG. 2, the noise reduction circuit includes a luminance signal input terminal 33, a subtracter 16 subtracting the input luminance signal from the output signal of a delay element 17, a factor-of-K multiplier circuit 18 multiplying the output signal of the subtracter 16 by the factor of K, an adder 19 adding the input luminance signal to the output signal of the factor-of-K multiplier circuit 18, and an output terminal 20 for the noise-reduced luminance signal.
Referring to FIG. 3, the noise reduction circuit includes a chromaticity signal input terminal 34, a subtracter 24 subtracting the input chromaticity signal from the output signal of an inverter 25 which inverts the output signal of a delay element 26, a factor-of-K multiplier circuit 27 multiplying the output signal of the subtracter 24 by the factor of K, an adder 28 adding the input chromaticity signal to the output signal of the factor-of-K multiplier circuit 27, and an output terminal 30 for the noise-reduced chromaticity signal.
The operations of the YC separation circuit and the noise reduction circuits having the structures described above will now be described.
The YC separation circuit shown in FIG. 1 utilizes the fact that the phase of a chromaticity component of a composite picture signal inverts in alternate frame or line periods. Referring to FIG. 1, the chromaticity component is cancelled in the adder 6 and roughly extracted in the subtracter 8. Finally, the chromaticity component is extracted in the band-pass filter 10 and appears at the chromaticity signal output terminal 31. On the other hand, the luminance component is obtained when the chromaticity component extracted in the manner described above is subtracted in the subtracter 12 from the composite picture signal. The luminance component thus obtained appears at the luminance signal output terminal 32. Then, in the noise reduction circuits shown in FIGS. 2 and 3, the S/N ratio is improved, utilizing the fact that there is a very strong autocorrelation between picture information supplied in one frame or line period and that supplied in the preceding frame or line period and that, in the case of random noise, the energy only of the noise component is lowered when the result of addition of the random noise appearing in those periods is averaged.
In the noise reduction circuit shown in FIG. 2, the luminance signal applied to the input terminal 33 in one period is subtracted in the subtracter 16 from the luminance signal which has been applied in the preceding period and in which the noise has been reduced, and the noise component appears at the output of the subtracter 16. The combination of the factor-of-K multiplier circuit 18 and the adder 19 averages the noise components appearing from the subtracter 16 in those periods thereby reducing the noise, and the noise-reduced luminance signal appears at the luminance signal output terminal 20. In the noise reduction circuit shown in FIG. 3, the inverter 25 is used to reduce noise in the chromaticity signal whose phase inverts in predetermined alternate periods. Thus, the noise in the chromaticity signal is reduced as in the case of the noise reduction in the luminance signal, and the noise-reduced chromaticity signal appears at the chromaticity signal output terminal 30.
In the manner described above, the noise-reduced luminance signal and the noise-reduced chromaticity signal can be extracted from the input composite picture signal.